A Brief History of the NOAA Very Long Baseline Interferometry Program

Starting in the mid 1970s, researchers from NOAA's National Geodetic Survey played a leading role in developing Very Long Baseline Interferometry stations and collecting observations that resulted in more accurate celestial and terrestrial reference frames. This pioneering work played an important role in increasing the fundamental understanding of our planet.

Photograph of the VLBI station at Fortaleza, Brazil, jointly developed
and operated by NOAA and the Brazilian Space Agency. Click image
for larger view.

Did you know that major weather systems like El Niño can actually cause
the Earth's rotation to speed up or slow down? In fact, one of
the effects of the 1997 El Niño weather system was to lengthen our day
by 0.6 milliseconds! Or did you know that the continents are in constant
motion? North America and Europe are drifting apart at a rate of about
1 centimeter per year. How were these discoveries made? The answer
might surprise you…

Much of what we know about our own planet has been gained by looking
to space. Astronomers use time differences in the arrival of microwave
signals from radio sources outside of our own galaxy (extragalactic)
to study the distant cosmos. This same technique, called Very Long Baseline
Interferometry or just "VLBI," can be used to study our
own planet and its place in the universe and to monitor the changes
in both. VLBI produces very precise distance measurements on the Earth's
surface, allowing us to learn about the Earth's size, its shape,
variations in its spin rate, changes in the orientation of its polar
axis—all by observing quasi-stellar objects (quasars) and other
natural radio sources.

For two decades, starting in the mid 1970s, researchers from NOAA's
National Geodetic Survey (NGS) played a leading role in developing VLBI
stations and collecting observations that resulted in more accurate
celestial and terrestrial reference frames. These reference frames
were used to make the first accurate measurements of the motions of
Earth's major tectonic plates and to monitor changes in the Earth's
orientation and length of day with greater resolution and accuracy.

NOAA no longer operates VLBI observatories nor participates in international
VLBI observing programs, but stations developed in Brazil, South Africa,
and Australia with NOAA support continue to operate, and VLBI continues
to be the single most important technique used by the International
Earth Rotation Service.

Historical Background

When the mathematician and physicist Leonhard Euler first reported
that his research on rotating bodies suggested that the Earth's
axis of figure might wobble slightly with respect to its axis of rotation,
Pierre LaPlace responded that "all of astronomy…depends…upon
the invariability of the Earth's axis of rotation…and
upon the uniformity of this rotation." If Euler's
research was correct, it would mean that lines of latitude would vary
by about 9 meters north and 9 meters south of their mean position
over a period of 10 months.

Despite LaPlace's skepticism, many of the leading astronomers
of the next century devoted substantial time and resources seeking to (unsuccessfully)
detect the variation of latitude.

In 1891, Seth Carlo Chandler, Jr., an insurance actuary, amateur
astronomer, and a former employee of the U.S. Coast Survey, stunned
the international scientific community by announcing his detection
of a variation of latitude at a period of 14 months. After further
analysis, Chandler determined that the "wobble" of the
Earth's axis is actually more complex, comprising at least annual
and 14-month oscillations and a long-term drift, and perhaps other
variations as well.

In 1878, the name of the U.S. Coast Survey was changed to the U.S.
Coast and Geodetic Survey (C&GS), and the variation of latitude
was certainly of interest to this agency charged with developing a
national geodetic control network for the United States. To
verify the variation of latitude, C&GS participated in an important
international observing program, sending a team to Japan in 1891 to
perform simultaneous observations with an International Geodetic Association
team. And, in 1899, C&GS joined in establishing and operating
the International Latitude Service (ILS). The ILS regularly
monitored polar motion, a more appropriate name for the "wobble," by
observing stars with "zenith telescopes." All observatories
observed the exact same stars, so that errors in the coordinates of
the stars tended to cancel out.

For more than 80 years, C&GS (and later NGS) continued to participate
in the ILS. Observatories in Gaithersburg, Maryland, and Ukiah, California,
(and for a short while in Cincinnati, Ohio) regularly submitted nightly
astronomic latitude observations to the central ILS Bureau in Mizusawa,
Japan.

The Introduction of VLBI

With the onset of the space age, it became clear that a more accurate
monitoring service was needed to support space navigation and modern
geodesy. Technologies developed for the exploration of space could
now be used to better monitor variations in the orientation of the Earth
in space, including not only polar motion, but also Universal Time,
precession, and nutation (a small periodic "nodding" of
the axis of rotation in space).

A team of National Aeronautics and Space Administration (NASA) and
Massachusetts Institute of Technology (MIT) researchers
began exploring the use of VLBI to measure motions of the Earth's
tectonic plates and Earth orientation. In VLBI, two or more radio
telescopes track natural sources, generally quasi-stellar objects
(called "quasars")
located great distances from Earth. The radiation received
from most quasars was emitted long before our solar system
was formed. Because the quasars are so far from Earth, they appear
to not be moving, thus forming a nearly inertial, or fixed, reference
frame, making them the obvious choice to use as a celestial reference
frame.

By using two or more radio telescopes to observe and record the signals
received from the same quasar at exactly the same time, scientists can
determine the time difference between the arrival of the signal at each
radio telescope. These differences can then be used to calculate
very precise distances and directions between the telescopes. VLBI
can determine distances between radio telescopes to within a millimeter
across an entire continent! Image courtesy of NASA.

However, also because the quasars are so distant, their signals
are extremely weak when they arrive at Earth, requiring
the use of large aperture radio telescopes (with collecting surfaces
typically tens of meters in diameter) with cryogenically cooled sensors.
Even with these powerful telescopes, the signals are so weak that
they are buried in noise. The noise and signals are recorded
on wideband digital tape recorders, with respect to highly precise
time tags provided by a hydrogen maser frequency standard located
at each observatory. At the completion of an observing session, typically
24 hours in length, the tapes are transported to a special
correlator center for processing. The noise is different on the tapes
recorded at each observatory, but the signal is the same, making
it possible for the correlator to determine the difference in time
of the arrival of signals between pairs of stations.

The delays in arrival times change as the Earth rotates. If
times are determined at several time periods for several quasars,
scientists can estimate very precisely the coordinates of the sources,
the differences in the clocks at the different stations, and the
baseline vectors (both magnitude and directions) of the lines between
observing stations.

With the third generation Mark III VLBI system developed by the
NASA-MIT team, the baseline lengths could be determined
to a few millimeters for stations separated by thousands of kilometers
and the orientation of the baseline could be determined to a fraction
of a millisecond of arc.

The POLARIS VLBI Network and Project MERIT

In 1977, NGS launched an initiative to establish an improved Earth
orientation monitoring system using VLBI. Project POLar-motion Analysis
by Radio Interferometric Surveying (POLARIS) involved the development
of three VLBI observatories, in a continental-scale open triangle, with
stations near Ft. Davis, Texas; Richmond, Florida; and Westford, Massachusetts.

One year later, at a working meeting held in Spain, the international
community decided to launch a project to explore the potential benefits
of establishing a new international Earth rotation service using a mix
of space techniques, most importantly, Very Long Baseline Interferometry,
Lunar Laser Ranging, and Satellite Laser Ranging. With Lunar Laser Ranging
and Satellite Laser Ranging, retroreflectors were placed on the moon
(by astronauts) and on artificial satellites. Telescopes fire pulses
of laser light to the reflectors, and the round-trip travel time of
the light is recorded. The one-way travel time multiplied by the speed
of light gives the distance. Measurements from different stations can
be used to determine the orbits of the satellites and moon, along with
the locations of the ranging stations on Earth.

Folded together, these space techniques were important components of
the new project, which was named Monitor Earth Rotation
and Intercompare Techniques (MERIT).

The POLARIS VLBI network was important in the deliberations leading
to the launch of project MERIT, because it assured that a regular series
of Earth orientation parameters would be available from VLBI. W. E.
Carter, NGS, was asked to serve as the VLBI technique coordinator for
project MERIT. NOAA, NASA, and the U.S. Naval Observatory (USNO) signed
an interagency agreement to collaborate on the application of VLBI to
Earth orientation. The collaboration between these three agencies
was known as the National Earth Orientation Service (NEOS) and resulted
in the building of a next-generation VLBI correlator center at USNO.

The success of project MERIT led to the establishment of the International
Earth Rotation Service (IERS). Today, the IERS regularly provides accurate
Earth Orientation Parameters (EOP) to the international scientific community,
using Lunar Laser Ranging, Satellite Laser Ranging, and Very Long Baseline
Interferometry, as well as other techniques, including use of the Global Positioning System (GPS).

Applying VLBI

VLBI soon proved to be the most powerful technique for maintaining
celestial and terrestrial reference frames and for providing
the highest accuracy and most complete suite of Earth orientation parameters.

To expand the international VLBI network, NGS made agreements with
agencies in Brazil, South Africa, and Australia. NGS provided
VLBI recording terminals, and in the case of Brazil,
a radio telescope. In
turn, the host nations agreed to operate stations near
Fortaleza, Brazil; Hartebeesthoek, South Africa; and Hobart, Tasmania,
Australia. These stations participated in observing sessions with the
POLARIS stations and also in observing sessions including stations developed
by other nations, most importantly Germany and Norway. Other VLBI observatories
were developed by NASA and USNO in Hawaii and Alaska,
and other nations built observatories, including Italy, Japan, Russia,
and China.

Plot of variations on the length-of-day determined from VLBI (black line)
and the change in atmospheric angular momentum (red line). Click image
for larger view and complete caption.

In the early 1980s, for a period of a few years, NGS also operated
two mobile VLBI units to monitor crustal motions in North
America and Europe. That work was eventually taken over by GPS, which
was less expensive and provided continuous 24-hour time series.

Conclusion

An unusual combination of a nearly century-old historical mission, the
need and means to improve the accuracy of the determination of polar motion
and other Earth orientation parameters by at least two orders of magnitude,
enlightened management, Congressional support, and collaboration with
NASA and USNO, all led to the extraordinary success of the NOAA VLBI program.
In fewer than 20 years, VLBI measurements verified plate tectonic theory,
created celestial and terrestrial reference frames never before thought
possible, compiled time series of Earth orientation parameters of unprecedented
accuracy and temporal resolution, and confirmed the deflection of electromagnetic
radiation in a gravitational field predicted by Einstein's theory
of relativity to a new order of magnitude.

Budget constraints ultimately resulted in the decision to terminate the
NOAA VLBI program in favor of GPS, which more directly addresses the highest-priority
operational responsibilities of NOAA. However, the VLBI correlator
built at the USNO continues to process data collected by a global network
of stations, many of which were in part inspired by project POLARIS. Ironically,
it is only because of the VLBI Earth orientation observations pioneered
by NOAA and still collected by other organizations around the world today
that NOAA is able to rely on GPS.

Contributed by William E. Carter, formerly with NOAA's National
Ocean Service, National Geodetic Survey, now at the University
of Florida